1. Field of the Invention
The present invention relates to circuitry used to provide a modulated gradient field in a nuclear magnetic resonance (NMR) detection system and to methods of modulating an applied gradient field.
2. Description of the Prior Art
In previous NMR detection systems, especially imaging systems, it is known to apply a modulated gradient field. For example, U.S. Pat. No. 4,165,479, issued to Mansfield, discloses both square and sinusoidal gradient modulation. Similarly, U.S. Pat. No. 4,322,684, issued to Hounsfield, discloses pulses of sinusoidally modulated signals applied to a gradient coil through a digital to analog converter. Other examples of NMR systems in which the gradient field is modulated include the following: U.S. Pat. No. 4,354,157, issued to Feiner, disclosing amplitude modulation using a cosine function; U.S. Pat. No. 4,280,096, issued to Karthe et al., in which the gradient coils are operated by discrete pulses; and U.S. Pat. No. 4,315,216, issued to Clow et al., in which the gradient pulse is a distorted sinusoid.
More recently, U.S. Pat. No. 4,384,255, issued to Young et al., discloses a particular circuit for providing pulses to gradient coils. The pulses applied in the disclosed system, however, are not sinusoidal, but rather are ramped.
None of the known prior art recognizes that a strong sinusoidal gradient field cannot be produced using known techniques without undesirable transient effects. Specifically, many cycles of sinusoidal oscillation may be necessary before a strong field of the desired amplitude will be obtained. In certain applications, however, such as high-speed NMR imaging, a strong gradient field sinusoidally modulated at audio frequencies is necessary. Although some of the known devices described above disclose pulsed sinusoidal gradients, it would be useful to have specific circuitry capable of providing such a gradient field without transients and additional methods for making optimal use of such circuitry.
In high-speed NMR imaging, information is collected from the imaged subject very rapidly, typically within several tens of milliseconds. The information collected, referred to herein as image data or imaging information, may represent a two or three dimensional spatial distribution. For this purpose, it is necessary to spatially encode the image data because the image data is ordinarily received as one dimensional time-varying data reflecting a parameter of the subject being imaged, such as water content. The necessary spatial encoding, in order to be meaningful, must also be performed at high speed, concurrently or interleaved with the image data collection. The spatial encoding is typically performed by gradient fields as explained in greater detail below.
Several design constraints apply to a high-speed NMR imaging system. If the image data is presented for display in a two-dimensional matrix format, for example, each element of the matrix will describe a finite volume element of the subject being imaged. Since the collected image data is time varying, it is desirable to represent each finite volume element by an associated band of frequencies .DELTA.f. From the basic equation defining the angular resonance frequency .omega. in terms of the gyromagnetic ratio .gamma. and the magnetic field B, EQU .omega.=.gamma.B, (1a)
we can obtain the relationship: EQU 2.pi..DELTA.f=.gamma.G.sub.x .DELTA.x. (1b)
In Equation (1b), .gamma. is a constant, G.sub.x is the magnetic field gradient in the x-direction, assumed to be the only gradient field applied, and .DELTA.x is the linear dimension of the finite volume element in the x-direction. This equation can be used to calculate the magnetic field gradient necessary to represent a finite volume of a particular size .DELTA.x over a frequency band of a particular width .DELTA.f. If the image is an N.times.N matrix, the total frequency span for the image will be N.sup.2 .DELTA.f. Typically, N may be 128.
The lower limit on the frequency band .DELTA.f for each finite volume element is determined by the field inhomogeneities. The ultimate natural limit will be 1/T.sub.2, where T.sub.2 is one NMR relaxation time. Using the above equations and assuming an image aperture of 40 cm, the total band width necessary for a 128.times.128 display matrix would typically be 1 MHz, and the field gradient required would be 6.25 gauss/cm. This is an extremely strong gradient field. The typical gradient fields available today achieve a maximum of 1 gauss/cm.
A further design constraint is that high-speed imaging is ordinarily performed using echoes, discussed in greater detail below. In some applications, these echoes are produced by the gradient field, which must therefore be modulated at audio frequencies such as several KHz. Because the typical gradient system now in use has a "ramp-up" time of one millisecond, an oscillation of the necessary frequency and amplitude is impossible to obtain. Furthermore, the power requirements for achieving an undistorted oscillation would be on the order of a megawatt, given the size of the gradient coils required.